Electron emissive coating material and method of application



United States Patent ELECTRON EMISSIVE COATING MATERIAL AND METHOD OF APPLICATION Mark N. Fredenburgh, Summit, N. 1., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application July 27, 1953, Serial No. 370,657

6 Claims.- (Cl. 20418l) The present invention relates to the art of electron emissive coating materials and methods of application, and particularly to an electron emissive coating material having a novel composition for advantageous application to a base by cataphoresis, and to an improved cataphoretic coating method.

In the preparation of directly heated filamentary oxidecoated cathodes for electron tubes, a filament core or base, which may be of tungsten or a nickel alloy for example, is usually coated with active materials by either (1) a drag process, (2) a spraying process, or (3) by the process of electrophoresis whereby charged particles of a coating suspension are directed to the filament when brought under the influence of an electric field. This process is known as cataphoresis when the particles of the suspension are positively charged and are directed to the negatively charged filament.

An electrophoretic method of coating has specific advantages over the methods of spray and drag coating, such as uniform centering of the filament, elimination of filament oxidation, improved coating smoothness and uniformity, process simplicity, and the ease of control during coating application. Normally, in order to obtain the necessary bonding of the coating after its application, the coated filament is passed through a solution of a resin, such a nitrocellulose. After drying, the coated filament is able to withstand the physical handling involved in further processing thereof.

In order to further improve this method of coating, it is desirable to combine the application of coating and binder into one operation, thus utilizing one bath only. Attempts to prepare a satisfactory suspension, whereby the coating and bonding takes place in one operation have heretofore been made by combining the nitro-cel lulose with the coating suspension. This combination, While successful to some extent, has been unsatisfactory due to such disadvantages as the inability to obtain the desired coating weight control, the production of tubes having inferior electrical characteristics due to the contaminating effects of the bonding material of the coating, and the inability to properly remove coating from emitter surfaces to be welded. In addition, the use of nitrocellulose in sutficient quantities necessary for successful bonding produces a negative charge on the carbonate particles which, in turn, necessitates the application of a positive potential to the filament. This results in erosion of the filament.

Due to the preparation of individual filaments from a continuously coated spool of filament, the filament must pass through an automatic machine which cuts and cleans the ends of the filaments so that the welding of the filament to the filament leads may be made. This handling requires a definite amount of bonding in the coating to withstand and the operations, but not so much as to prevent making its removal a positive operation. On a drag coated filament, the bond is usually obtained by an alkaline earth salt which has sufficient brittleness to react to ICC pressure or friction of moderate intensity to produce a clean area. The use of an organic resin like nitrocellulose, for bonding, produces a more elastic type of bond due to the fibrous structure of the nitrocellulose, and this bonding agent is not therefore characterized by the desirable reaction to pressure or friction aforementioned.

Most materials, when added to the emitting mixture for the purpose of bonding the particles, if allowed to remain in the finished tube during its manufacture, will have an adverse effect on the tube characteristics. In the case of drag coating, where alkaline earth nitrates are used for bonding, no adverse effects are experienced since these materials decompose during the tube manufacture into alkaline earth oxides, the same compounds resulting from the decomposition of the alkaline earth carbonate used for obtaining the emitting surface.

When materials such as organic resins are used for bonding, the decomposition of these materials during the heat treatment may produce products having adverse effects on the tube characteristics. These adverse effects will depend primarily on the amount of material decomposed and it should be pointed out that in the effective bonding of the filament in a one bath suspension the amount of resin needed is considerably in excess of that required for the bonding of cathodes where the carbonates are applied by spraying and handling resistance is reduced to a minimum.

When nitrocellulose is used in the coating suspension, it has been found that, due to the formation of residual carbon in the coating, and other by-products from the decomposition which may deposit on tube parts, adverse effects such as low emission or low mutual conductance may result.

Furthermore, when coating supsensions containing nitrocellulose are used in a cataphoretic process, it has been found that coatings having a low weight per given area are not practical as the coating voltage range necessary for application does not cover a uniform weight range starting from zero.

It is therefore an object of the invention to coat continuous filament stock with alkaline earth carbonates by a continuous one bath cataphoretic process, capable of producing directly heated oxide-coated emitters of superior physical and electrical characteristics.

Further objects are to provide a suspension useful in a cataphoretic coating method, of electron emissive materials containing a binder, having the advantages of (1) stability of the suspension, (2) preservation of the normal positive charge on the coating material particles during coating, (3) providing a coating having desirable adherence to withstand normal handling without coating loss, (4) providing a coating that is easily removed from surfaces to be free of the coating, (5) complete elimination of the binder before or during exhaust of an electron tube in which a cathode coated according to the invention is used, (6) and complete coating weight control from .2 to 10 mgs./cm.

A feature of the invention involves the use of resins such as polymeric esters of methacrylic acid, as the binder in a suspension of electron emissive materials. For example, the use of polymeric methyl methacrylate as a binder gives a type of bond similar to that of salts used in the drag process aforementioned, and its properties may be controlled by the addition of a low vapor pressure sol-..

process mentioned, applicant has discovered that the use:

of resins such as the polymeric esters of methacrylic acid, when used as the bonding material in an electrophoretic suspension, Will provide an advantageous coating material for use in a continuous one bath process.

In the case of a resin such as. polymeric methyl methacrylate, no adverse eflectssuch as those obtained from the use of nitrocellulose have been observed when used in the amounts necessary for satisfactory bonding. These results are assumed to be due to the fact that no decomposition of this resin takes place during the tube processing. Instead, the solid resin depolymerizes, the fluid monomeric ester is formed, and due, to the heat present, is immediately removed during tube manufacture from the cathode or other tube parts by vaporization since the vapor pressure of the ester is approximately the same asthat of water.

The resin according to the invention will not only tend to stabilize the suspension and maintain the normal positive charge of the particles during coating by cataphoresis, but will produce 1) the correct type of adherence necessary for all handling of the filament without preventing an easy removal of the coating from those surfaces which must be free of coating, (2) satisfactory tube characteristics due to the complete elimination of the resin before or during exhaust, and (3) complete coating weight control from .2 to mgs./cm. over a very wide voltage range.

Furthermore, when suspensions containing polymeric methyl methacrylate are used in a cataphoretic coating method, coating starts as soon as voltage is applied and increases in a uniform manner as the voltage is increased.

According to one example for practicing the invention, a coating suspension is prepared as follows:

SUSPENDING MEDIUM Measure out 3,000 milliliters of acetone and 105 milliliters of dibutyl phthalate and pour into a glass bottle of 1 gallon capacity.

Weigh out 210 grams of polymeric methyl methacrylate powder, add to the suspending liquids, agitate immediately by hand, then transfer to a rolling machine and roll until the methacrylate is completely dissolved.

Adjust the volume of the solution to a total of 3500 milliliters by the addition of acetone.

CARBONATES Weigh out 525 grams of barium, strontium, and calcium carbonates in the relative amounts by weight of 57% barium carbonate, 39% strontium carbonate, and 4% calcium carbonate. Instead of mixing separate carbonates, it is feasible to coprecipitate the carbonates to form a single carbonate containing the same relative amount of elements indicated above. Where the carbonates referred to are prepared by the interaction of barium, strontium and calcium nitrates and sodium carbonate, they usually contain a small amount of sodium nitrate which is a soluble ionizable salt. When the carbonates are prepared in other ways, leaving no residue of soluble ionizable salts such as sodium nitrate, such salts should be added to the carbonates referred to in the amount of .05% by weight for the purposes of the instant example. The carbonates including the soluble ionizable salt or salts referred to are heat treated for two hours at from 130 to 150 C.

PREPARATION OF COATING SUSPENSION Place the 525 grams of the alkaline earth carbonates and the sodium nitrate into a clean dry porcelain ball mill jar of 1.25 gals. capacity and containing 2500 grams of porcelain balls 4 in diameter. Add 1000 ml. of the suspending medium, close the jar and rotate at aspee'd of 60-70 R. P. M. for a period of hours.

After milling, open the jar, add 2500 milliliters of suspending medium, close the jar, androtate mi1l again for aperioid of 1 hour.

COATING APPLICATION Example 1 In the application of the described suspension for the 5 coating of a filament of .001" dia., the amount of coating applied, for example, by a coating machine described in copending application Serial No. 426,874, filed April 29,

1954, by applicant, and operated at a speed of 5 meters per minute, is represented by the following data:

Coating Coating Wt., Current, Mgs. per Coating Voltage Me. 200 mm.

' length Example 2 Coating Coating Wt., Current, Mgs. per Coating Voltage Me. 200 mm.

length Example 3 Coating Coating Wt., Current, Mgs. per Coat ng Voltage Ma. 200 mm length Example 4 The negative influence on the efiiciency of the applied coating by the addition of approximately .2% by weight of calcium nitrate to the described suspension is shown 55 by the following data:

Coating Coating Wt., Coating Voltage Current, Mgs. per 200 Ma. mm. length THE SOLUBLE IONIZABLE SALT Y In the use of the suspensions described, I have also discovered thatthe most stable suspensions (i. .e. those having the slowest settling rates), produce the highest efliciency with respect to the total amount of coated filament which can be produced from a given quantity of suspension used under a given set of coating conditions. This, in turn, has been found to be due to the percentage of soluble ionizable salts remaining in the alkaline earth carbonates that is, barium, strontium and calcium carbonate powders, from their manufacturing process.

One process of manufacture of the carbonate powder aforementioned comprises precipitating a barium, strontium, calcium, nitrate solution with a solution of sodium carbonate. This reaction produces an amount of sodium nitrate, which is a soluble ionizable salt, small amounts of which remain in the resultant carbonate powder after washing. The final amount of such sodium nitrate depends upon the efliciency of the washing procedure following the precipitation. Normally, the sodium nitrate content of a carefully made and thoroughly washed carbonate precipitate, will be around .05% by weight of the carbonate powder.

Applicant has found that the amount of soluble ionizable salt in the carbonate powder should be confined within the range of from .01% to 0.5% by weight of the powder; The purposes served by the soluble ionizable salt according to the invention are to control the dispersion of the solids, i. e., the carbonate particles, in the suspension; and to provide desired charges on the particles required for a cataphoretic application thereof. The aforementioned range of salt content is critical for accomplishing these purposes. I

Thus, if the amount of soluble ionizable salt is below .01%, the lower limit of the range according to the invention dispersion is at a maximum which may result in quality defects, such as bare spots or uneven coating. If the amount of soluble ionizable salt is above .5 it induces excessive agglomeration of the coating particles with substantial loss in coating efficiency.

In addition to sodium nitrate, applicant has found that other soluble ionizable salts, such as barium nitrate, calcium nitrate, strontium nitrate and ammonium nitrate among others produce equally good results.

As indicated above, some sodium nitrate remains'in the carbonate powder from the process of manufacture aforementioned, the maximum amount being about .05 In some instances, however, the final amount of sodium nitrate may be appreciably less that .05% and in fact below .01%, the lower limit of the acceptable range according to the invention as stated before herein. In this event, it becomes necessary to add a soluble ionizable salt to bring the amount thereof within the range of from .01% to .5% and preferably to the optimum amount of around .05

THE METHACRYLATE CONTENT OF THE SUSPENSION The methacrylate content of the suspension, as before mentioned, may be varied for filaments having different diameters or cross-sections, to provide the bonding requirements of a particular filament. However, it should not be included in amount greater than grams per 100 milliliters of the suspension. The optimum amount of the methacrylate found satisfactory for coating all types of filaments without regard to their diameters or crosssections, is 8 grams per 100 milliliters of suspension. The lower limit of methacrylate content according to the invention is four grams per 100 milliliters of suspension. As the methacrylate content is increased from 4 grams to 10 grams per 100 milliliters of suspension, the bonding effect of an applied coating is, of course, also increased. Applicant has found that when the methacrylate is present in an amount of 10 grams per 100 milliliters of suspension, a coating made cataphoretically from the suspension has the bonding properties necessary for any filament application.

As the methacrylate content is reduced from 10 grams per milliliters of suspensioma point is reached where the bonding action becomes so weak that practical handling of a coated filament without causing coating defects, becomes impossible. This point is reached when the methacrylate content is reduced to less than around four grams per 100 milliliters of suspension.

While the various polymeric esters of methacrylic acid, such as methyl, ethyl, normal propyl, normal butyl and isobutyl may be utilized for satisfactory results, it has been found that for the specific purposes mentioned, the methyl ester has a combination of properties superior to others in the group. For instance, the hardness of the polymeric methyl ester is greatest, and therefore, by the introduction of a compatible solvent of low vapor pressure such as dibutyl phthalate in an amount from 0 to 50% by weight of the methacrylate, this hardness may be controlled. Also, on depolymerization the methyl ester produces the monomeric ester having a relatively low vapor pressure, which, in turn, is removed from the tube or tube parts at a relatively low temperature during tube processing.

THE METHOD OF THE INVENTION In the foregoing four examples of a coating application by cataphoresis using the novel coating material of the invention, there is presented four sets of conditions including filament diameter, suspension composition, coating voltage, coating current and coating weight.

In applying the coating under the aforementioned conditions, the coating suspension was contained in a single coating tank. The filament to be coated was continuously passed through the suspension in the tank over suitable pulleys at a speed of five meters per minute. Suitable connection of the filament to a D. C. electrical source of negative polarity of 0300 volts was effected. An electrode was extended into the suspension serving as an anode. The voltages and currents indicated in the aforementioned examples were across the anode and filament referred to through the suspension.

It was found that the continuous passage of the filament through the suspension and the nature of the suspension according to the invention, permitted very accurate control of coating weight from .2 to 10 milligrams per square centimeter. Applicant found that coating weight could be very accurately related to the voltage used. Thus, under the conditions specified in Example a potential difference of 40 volts between the electrode aforementioned and the filament, produced a coating weight of .94 milligram on a filament surface 200 millimeters long and having a diameter of .001 inch. As the voltage was increased to 100 volts, the coating weight increased to 1.74 mgs. on the same surface area.

In Example 3, a ribbon filament about .040 inch wide was coated under conditions differing from those of EX- ample 1, only with respect to applied voltage. It will be noted that with a voltage of 25 volts a coating Weight of 11.5 milligrams was applied per 200 millimeters of ribbon length. This length, of course, provided a much larger arear than the similar length aforementioned of the filament having a diameter of .001 inch. As the voltage was increased -to volts, the coating weight increased to a value of 32.0 milligrams per 200 millimeters of ribbon length.

In each case, it will be observed that the coating Weight increases substantially linearly with voltage increase.

Examples 2 and 4 show the effect of adding more soluble ionizable salts to the suspension used in Examples 1 and 3, which included about .05% of such salts by weight of the carbonate. The conditions in Example 2 are the same as in Example 1, except that .05% more salt is added to the suspension, so that the total amount of ionizable salts therein is .10%. While coating weight control by the applied voltage s still feasible, agglomeration of the particles in the suspension has proceeded to such a degree that the coating efficiency of the suspension has been affected and higher voltages are required for the same weight.

In Example 4, the only change from the conditions in Example 3 is the addition of a further .2% of ionizable salts. This makes the total amount of such salts in Example 4, 25% by weight of the carbonate in the suspension. Such increase in the salt content, it will be noted, requires an appreciable increase in the voltage to produce the same coating weight.

In all the Examples referred to, the speed of the base to be coated through the coating suspension was meters per minute. This speed is preferred when ribbons such as were used in Examples 3 and 4 or larger filaments, are to be coated. A speed of meters per minute is preferred for fine filaments such as were coated in Examples 1 and 2.

Feeding the base at a lower rate than 5 meters per minute produces no quality disadvantage but reduces manufacturing efiiciency.

The maximum speed of coating will be that speed at which the required coating Weight and coating quality are obtained and will vary with filament of different size and shape.

Adjacent the tank referred to was disposed a furnace heated, for example, by resistance elements, and having a passageway through which the coated filament after emerging from the suspension in the tank, was adapted to pass. The function of the furnace was to vaporize and drive off volatile components of the suspension, such as the acetone used therein. After such heating at 200- 300 C., the coated filament was dry and permitted practical handling incidental to further processing, without loss of or harm to thecoating. Such incidental handling involved cutting the filament to lengths and the application of pressure as by impact means to portions of the lengths to remove'the coating cleanly for welding to tube elements such as leads.

It will be noted from the foregoing that applicant has provided a novel coating suspension that permits a one bath coating application by cataphoresis with reduced harmful effects during ultimate utilization of a base so coated, such as a coated filamentary cathode used in an electron tube. This advantage is accomplished by adding a predetermined amount of polymeric ester of methacrylic acid to the suspension for service as a binder. It

avoids the disadvantage incidental to the prior use of' nitrocellulose as the binder. The novel coating suspension of the invention also permits an accurate control of coating weight by the voltage applied, which was impossible to accomplish heretofore. This control is efficiently feasible because of the presence of a predetermined amount of a soluble ionizable salt in the suspenslon.

Both the methacrylate and the ionizable salt used in the coating suspension contribute to an improved method of manufacture of electron emissive filamentary cathodes and provide cathodes characterized by superior operatlon in an electron tube.

' What is claimed is: I

1. A coating suspension of electron emitting material adapted for application to a base having a negative polarity, said suspension comprising a polymeric ester of methacrylic acid, an ionizable salt, particles of said emissive material, and a polar solvent for said polymeric ester and said salt consisting of acetone, said polymeric ester in solution in said solvent contributing to the formation of positive charges on said particles for migration of said particles to said base during a coating operation, said polymeric ester converting to a monomeric phase with a vapor pressure approximately that of water at a first predetermined temperature used in the manufacture of electron tubes and a gaseous phase at a second predetermined temperature used in said manufacture, whereby said ester is adapted to be removed during the processing of a tube having an element thereof coated with said suspension and to preserve said tube from contamination.

2. A coating suspension of electron emitting material adapted for advantageous use in a cataphoretic coating operation wherein the base to be coated is caused to have a negative polarity for reduced erosion of said base, said suspension comprising particles of emitting material, a binder consisting of a polymeric ester of methacrylic acid, an ionizable salt, and a polar solvent consisting of acetone in which said polymeric ester and said salt are in solution, said polymeric ester, salt and polar solvent contributing to the formation of a positive charge on said particles which persists when said suspension is disposed in an electric field produced between said base of negative polarity and an electrode of positive polarity during a coating operation, said particles being adapted to form a coating mass up to 10 milligrams per square centimeter, for providing a coating of improved bonding quality.

3. A coating suspension of electron emissive material adapted to be applied by cataphoresis to provide a coating having a mass up to 10 milligrams per square centimeter and characterized by improved bonding to a base and involving: reduced erosion of the base, said suspension consisting of particles of said material, a polymeric ester of methacrylic acid, an ionizable salt, and a polar solvent in which said polymeric ester and said salt are dissolved, the amount of said polymeric ester being from four to ten grams in each milliliters of suspension, the amount of said salt being from 0.01 to 0.5% by weight of said particles, said particles being adapted to receive a positive charge persisting during a coating operation for producing said coating mass when a base having a negative polarity and an electrode having a positive polarity are disposed in said suspension.

4. A coating suspension of electron emissive material adapted to be applied to a base by cataphoresis to provide a coating having a thickness to provide a mass thereof up to 10 milligrams per square centimeter, said suspension consisting of particles of said material, a polymeric ester of methacrylic acid, the amount of said polymeric ester in relation to the amount of said particles being by weight from to 350 grams of said ester, to 525 grams of said particles for providing improved bonding of said particles to said base, an ionizable salt, and a polar solvent in which said polymeric ester and said salt are dissolved, said particles being adapted to receive a positive charge persisting when an electric field is produced between the base having a negative polarity and an electrode having a positive polarity in said suspension, for causing said particles to migrate to said base with reduced erosion of said base to provide saidcoating mass.

5. Method of coating a metal filament by cataphoresis withelectron emissive material to form a coating having a mass of from 0.2 to 10.0 milligrams per square centimeter, comprising continuously passing successive portions of said filament through a suspension containing said emissive material, a polar solvent, a polymeric ester of methacrylic acidand an ionizable salt, and with said polymeric ester and said salt dissolved in said solvent, producing a voltage difference between said filament and an electrode in said suspension by causing said filament to have a negative polarity with respect to said electrode, whereby said particles have a positive charge and migrate to said filament to provide said coating mass, heating said portions after leaving said suspension to drive ofi said solvent, whereby said coating is characterized by improved adherence and is sufficiently brittle for mechanical removal from portions of said filament.

6. Method of making a coated cathode having a coating mass of from 0.2 to 10 milligrams per square centimeter, comprising passing the base stock of said cathode through a single bath containing particles of electron emissive material, a polymeric ester of methacrylic acid, an ionizable salt and a polar solvent in which said polymeric ester and said salt are dissolved and in which said particles are suspended, while impressing a negative polarity on said base stock and a positive polarity on an electrode in said bath, drying the coated base stock to drive 01f said solvent, severing said coated base stock to provide a coated cathode, mounting said cathode in an electron tube, and heating said cathode in said tube to a temperature to cause said polymeric ester to change first to a liquid phase, and then to a gaseous phase for ready removal from said tube of substantially all of said polymeric ester in the coating on said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 10 2,393,068 Ruben Jan. 15, 1946 2,442,863 Schneider June 8, 1948 2,462,125 Oakes Feb. 22, 1949 FOREIGN PATENTS 914,190 France June 11, 1946 OTHER REFERENCES Plasticizers (Buttrey), published by Cleaver-Hume 10 Press Ltd., 1947; pp. 9 and 10 relied on.

Vinyl and Related Polymers (Schildknecht), published by John Wiley & Sons, 1952, N. Y.; page 217, 2nd para, relied on.

line 38, for "approximately, 04

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION July 23, 1957 Patent No. 2,800,446

Mark N. Fredenburgh It is hereby certified that error appears iii the printed specification of the above numbered patent requiring correction and that the said Let oers Patent should read as corrected below.

; column 4,

Column 1, line 69, after "withstand" strike out "and" 0 O" read approximately ,040 column 6, line 59, for "arear read area Signed and sealed this 1st day of October 1957.,

Attest: KARL a H WINE ROBERT C. WATSON Comnissioner of Patents Attesting Officer 

1. A COATING SUSPENSION OF ELECTRON EMITTING MATERIAL ADAPTED FOR APPLICATION TO A BASE HAVING A NEGATIVE POLARITY, SAID SUSPENSION COMPRISING A POLYMERIC ESTER OF METHACRYLIC ACID, AN IONIZABLE SALT, PARTICLES OF SAID EMISSIVE MATERIAL, AND A POLAR SOLVENT FOR SAID POLYMERIC ESTER AND SAID SALT CONSISTING OF ACETONE, SAID POLYMERIC ESTER IN SOLUTION IN SAID SOLVENT CONTRIBUTING TO THE FORMATION OF POSITIVE CHARGES ON SAID PARTICLES FOR MIGRATION OF SAID PARTICLES TO SAID BASE DURING A COATING OPERATION, SAID POLYMERIC ESTER CONVERTING TO A MONOMERIC PHASE WITH A VAPOR PRESSURE APPROXIMATELY THAT OF WATER AT A FIRST PREDETERMINED TEMPERATURE USED IN THE MANUFACTURE OF ELECTRON TUBES AND A GASEOUS PHASE AT A SECOND PREDETERMINED TEMPERATURE USED IN SAID MANUFACTURE, WHEREBY SAID ESTER IS ADAPTED TO BE REMOVED DURING THE PROCESSING OF A TUBE HAVING AN ELEMENT THEROF COATED WITH SAID SUSPENSION AND TO PRESERVE SAID TUBE FROM CONTAMINATION. 